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  • Truck Trend Shop Class: Engine Compression

Truck Trend Shop Class: Engine Compression

Compression

Alex Steele
May 30, 2019
Compression: An automotive term used by everybody, forever. Let's get into some detail and maybe develop a better understanding.
Pressure, and more pressure: It's a must-have to produce power within an internal-combustion engine. Lighting up a mixture of air and fuel is a wonderful thing, but if the mix is not fired under considerable pressure in a confined space, it will produce heat and emissions, but not the force needed to rotate a crankshaft. A mild comparison might be your home furnace keeping you warm and fuzzy at night and a supercharged big-block lifting the front wheels off the ground on takeoff. Maybe not so mild, but you get the idea.
Heat is generated by compression and its subsequent increase of pressure. This aids in the vaporization of fuel within the air/fuel mix. A larger number of smaller droplets of fuel produce more surface area than a smaller number of larger droplets. This larger area increases the fuel's contact with oxygen (needed for fuel to burn), therefore enhancing the expansion of gases during combustion. This process increases thermal efficiency, meaning the expansion of gases during the burn produces more mechanical energy that drives the piston down and less energy wasted as heat out the tailpipe.
Photo 2/3   |   Truck Trend Shop Class Engine Compression Piston
Static Compression
As we know, the bigger an engine, the more power it produces; in the same sentence, higher compression of air and fuel can increase power even further.
Static compression is what engineers, engine builders, and even do-it-yourselfers dial in to an engine build. Basically, it's the exact compression ratio formulated into engine construction in a perfect-world environment.
We'll start with engine displacement, measured in cubic inches, cubic centimeters, or liters—measurements of volume (space). We're not going to get into the mathematical equations, but using the cylinder bore diameter and stroke of the crankshaft (length between bottom-dead-center, BDC, and top-dead-center, TDC, at the connecting rod), the volume of a cylinder is calculated. Multiplying that figure by the number of cylinders gives you engine displacement.
Compression ratio is the difference between the total volume of the cylinder and combustion chamber at BDC and their volume at TDC. If you have 1,000 cc of space at BDC and 100 cc at TDC, the compression ratio is 1,000:100, or 10:1.
Hopefully, everybody's catching on to the fact that engine displacement does not determine compression. It's calculated by factors including the contours of the piston crown (top surface), deck clearance (distance between top of the piston and block deck), of course the combustion-chamber volume of the cylinder head, the thickness of the head gasket, and even the clearance between the piston and cylinder wall above the top ring.
Dynamic Compression
A dynamic compression ratio is basically the same thing as static (the more commonly used number), only this time we're taking valve timing into consideration and being a bit more accurate to conditions of a running engine. With static compression calculations, the cylinder is considered completely sealed (intake and exhaust valves fully closed) at BDC, meaning the air is being compressed immediately when the piston begins traveling upward on its compression stroke. In fact, that's not usually the case. When cam timing dictates the intake valve closed after-bottom-dead-center (ABDC), actual compression of air/fuel does not begin until that point. Therefore, dynamic compression will always be less than static.
Dynamic compression was originally a fixed value on production engines, until variable-valve timing changed all that.
Photo 3/3   |   Truck Trend Shop Class Engine Compression Pistons Plastic
Cylinder Pressure
After going over design and calculations determining compression ratios, cylinder pressure is the real deal: How much pressure, in pounds per square inch, is squeezed into the combustion chamber at TDC.
We're taking static compression, modified by dynamic compression (valve timing), and adding a slew of additional factors that affect actual psi. Intake and exhaust system design, throttle-body diameter, throttle position, engine rpm, and more all play a role in the flow of air being compressed.
Compression Test
A better name might be cylinder-pressure test.
A rough estimate of cylinder pressure is 15 to 20 times the compression ratio. So 10:1 should produce about 150 to 200 psi. Production-engine manufacturers typically provide a specification or a range for testing.
A static gas-engine compression test—static here means non-running and doesn't necessarily refer to static compression—requires a pressure gauge with a hose and sealed fitting to screw into each spark-plug hole. Remove all plugs, install the gauge at one cylinder, disable fuel and spark, hold the throttle wide open, and crank the engine about four puffs (compression strokes of the tested cylinder). Repeat on all cylinders and record readings. Squirt a small amount of motor oil in all cylinders, repeat testing, and record again.
If the dry-to-wet comparison shows a significant increase in pressure wet, worn piston rings may be at fault. The oil improves the piston ring-to-cylinder wall seal temporarily.
Just as important as the pressure readings being within specification is an equal balance. There should be no more than a 10-percent difference between the high and low cylinder.
Test example: All cylinders produced 175 psi but one, which made 100 psi, and the wet test had little effect on the low cylinder. We can assume piston rings are not the problem and can then lean toward a leaking intake or exhaust valve.
In real-word production-car service, when diagnosing a misfire caused by an internal engine failure, oftentimes a compression test gets bypassed.
A cylinder leakdown test is often a more efficient method of narrowing down the problem more quickly. The tool uses two gauges and compressed air. One gauge reads the applied air pressure (100 psi) and the other uses a 0 to 100 psi or percentage scale that's manually zeroed.
The test cylinder is brought to TDC of its compression stroke, and the hose (just like compression test) is installed in the spark-plug hole. When the hose is connected to the tool, 100 psi of air pressure is applied inside the compression chamber (simulating compression pressure). The second gauge will read the percentage or pounds of pressure being lost (leaking) at that cylinder.
Even a perfect-condition engine will show a small amount of leakage that normally gets by the piston rings, and that will increase with normal high-mileage wear.
Test example: The suspect cylinder with low compression at 100 psi shows 50-percent leakage. Listening, feeling, and/or smelling air coming out through the exhaust or intake will confirm excessive leakage at an exhaust or intake valve, respectively.
Another advantage to a leakdown test is detecting a bad head gasket or cracked cylinder head. While pressure is applied to the cylinder, the coolant level at the radiator will rise, or bubbles will appear, confirming a compression leak into the cooling system.
Detonation!
There's that word again: detonation—aka engine knock or ping. It's the internal-combustion anomaly that produces a horrendous metallic rattle on acceleration.
When an air/fuel mixture is compressed and ignited at the spark plug, the flame front spreads outward uniformly and provides a nearly complete burn of the fuel while keeping combustion-chamber pressure and temperature in check.
Detonation is the effect when the spark is not the only ignition point. Pockets of air/fuel elsewhere in the cylinder ignite and produce flame fronts of their own, after the spark plug fires. The results are unwanted shock waves and extreme spikes in combustion pressure and temperature. If the knock is bad enough and takes place for a long enough time, detonation will damage an engine—meltdown of pistons is often the case.
The tricky part about detonation is that it takes place right at the borderline of peak combustion efficiency and power. In certain applications, small amounts of knock can be lived with and controlled.
There are several instigators to detonation, but all involve high pressure, temperature, and unwanted ignition.
Timing
On a four-stroke gas engine, ignition timing is crucial to get the optimum mechanical power out of a burn in the combustion chamber. The idea is advancing the timing before TDC to the right spot. As the piston approaches TDC, the ideal ignition point is before it gets there. This way the burn begins early, and the peak of combustion actually takes place a couple of degrees after TDC. This provides full force of the explosion driving the piston down.
Too much advance of the spark means peak combustion happens before TDC, and the result is detonation. Not enough advance means the peak is too far after TDC, hampering the burn and not efficiently utilizing the full power stroke of the crankshaft.
Ideal combustion is achieved when spark timing is advanced right to the point detonation begins, and then maybe backed off (retarded) a couple of degrees. That's a focal point of performance tuning.
Octane
Higher compression forces an air/fuel mixture to ignite at a lower temperature. Therefore, higher-octane fuel is required, which ignites at a higher temperature. Too low an octane to meet the needs of engine compression will cause detonation.
Octane ratings are basically the point of a fuel's detonation under specific compression ratios.
Along with ignition timing, octane, and compression, engine knock can be attributed to high engine coolant temperature, intake air temperature, or a lean air/fuel mixture.
Knock Control
Late-model engine designs have come a long way in the engineering of pistons and cylinder heads to improve combustion efficiency, allowing higher-compression engines to run on regular gas. Direct injection and variable-valve timing also play parts in dealing with detonation.
Dynamic PCM control of ignition timing and fuel injection helps retain combustion efficiency and reduce knock, with some assistance from "knock sensor" data inputs, which make relevant adjustments possible.
Exhaust gas recirculation (EGR), in different forms, has been used since way back when to both reduce oxides of nitrogen emissions and suppress detonation by adding exhaust gases to the air/fuel mix, which cools the combustion process.
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